Communication device and SRS transmission control method

ABSTRACT

Provided are a communication device and an SRS transmission method capable of reducing the possibility of a difference in recognition between the presence or absence of an SRS transmission between a base station and a terminal or of an SRS resource so as to prevent degradation of system throughput. At a terminal ( 200 ), a reception processing unit ( 203 ) detects control information indicating whether or not to request transmission of a sounding reference signal (SRS), whereupon a transmission signal forming unit ( 207 ) transmits an A-SRS by way of control by a transmission control unit ( 206 ) on the basis of control information. The transmission control unit ( 206 ) determines whether or not to execute SRS transmission on the basis of an “SRS Transmission Execution Rule” and the reception status of trigger information.

BACKGROUND

1. Technical Field

The present invention relates to a communication apparatus and an SRStransmission control method.

2. Description of the Related Art

3rd Generation Partnership Project Radio Access Network Long TermEvolution (3GPP-LTE) (hereinafter, referred to as “LTE”) employsOrthogonal Frequency Division Multiple Access (OFDMA) as the downlinkcommunication scheme and employs Single Carrier Frequency DivisionMultiple Access (SC-FDMA) as the uplink communication scheme (see,Non-Patent Literatures (hereinafter, abbreviated as NPL) 1, 2 and 3, forexample). In addition, a Periodic Sounding Reference signal (P-SRS) isused in the uplink of LTE as a reference signal for measuring uplinkreception quality.

In order for terminals to transmit the P-SRS to a base station, an SRStransmission subframe shared by all terminals (hereinafter, referred toas “common SRS subframe”) is configured. This common SRS subframe isdefined by a combination of a predetermined periodicity and a subframeoffset on a per cell basis. The information about the common SRSsubframe is broadcasted to the terminals in the cell. For example, whenthe periodicity is 10 subframes, and the offset is 3, a third subframein a frame (formed of 10 subframes) is configured as a common SRSsubframe. In the common SRS subframe, all the terminals in the cell stopthe transmission of data signals at the last SC-FDMA symbol of thesubframe and use this period as a transmission resource for referencesignals.

In addition, each terminal is specifically configured with an SRStransmission subframe by a higher layer (RRC layer above the physicallayer) (hereinafter, referred to as “specific SRS subframe”). Eachterminal transmits a P-SRS in the configured specific SRS subframe. Inaddition, each terminal is configured with parameters on the SRSresource (hereinafter, may be referred to as “SRS resource parameter”)and also notified of the parameter. The parameters on the SRS resourceinclude the bandwidth and band position of the SRS (or position whereSRS band starts), Cyclic Shift, Comb (which corresponds toidentification information on the subcarrier group), and/or the like.The terminal transmits an SRS using the resource in accordance with thenotified parameters. In addition, SRS frequency hopping may beconfigured in some cases.

In addition, the introduction of a dynamic aperiodic SRS (hereinafter,referred to as “A-SRS”) into the uplink of LTE-Advanced, which is anadvanced version of LTE (hereinafter, referred to as “LTE-A”), has beendiscussed. The transmission timing of an A-SRS is controlled by triggerinformation (e.g., 1-bit information). This trigger information istransmitted to a terminal from a base station on a physical layercontrol channel (i.e., PDCCH) (e.g., see NPL 4). More specifically, theterminal transmits an A-SRS only in response to an A-SRS transmissionrequest made by the trigger information (i.e., A-SRS transmissionrequest). In addition, studies have been carried out on defining, as theA-SRS transmission timing, the first common SRS subframe located at orafter a k-th subframe (e.g., k=4) from the subframe in which the triggerinformation is transmitted. As described above, although a P-SRS istransmitted periodically, it is possible to cause a terminal to transmitan A-SRS frequently within a short period only during a data burst inthe uplink transmission, for example.

Moreover, LTE-A provides control information formats for various typesof data assignment reporting. The control information formats in thedownlink include: DCI format 1A for allocation of resource blocksconsecutive in number (Virtual RBs or Physical RBs); DCI format 1, whichallows allocation of RBs not consecutive in number (hereinafter,referred to as “non-contiguous bandwidth allocation”); DCI formats 2 and2A for assigning a spatial-multiplexing MIMO transmission; a downlinkassignment control information format for assigning a beam-formingtransmission (“beam-forming assignment downlink format:” DCI format 1B);and a downlink assignment control information format for assigning amulti-user MIMO transmission (“multi-user MIMO assignment downlinkformat:” DCI format 1D), and/or the like. Meanwhile, the uplinkassignment formats include DCI format 0 for assigning a single antennaport transmission and DCI format 4 for assigning an uplinkspatial-multiplexing MIMO transmission. DCI format 4 is used for onlyterminals configured with an uplink spatial-multiplexing MIMOtransmission.

In addition, DCI format 0 and DCI format 1A are adjusted in size bypadding so that each format consists of the same number of bits. DCIformat 0 and DCI format 1A are also called DCI format 0/1A in somecases. DCI formats 1, 2, 2A, 1B and 1D are used depending on thedownlink transmission mode configured for each terminal (i.e.,non-contiguous bandwidth allocation, spatial-multiplexing MIMOtransmission, beam-forming transmission or multi-user MIMO transmission)and are configured for each terminal. Meanwhile, DCI format 0/1A can beused independently of the transmission mode and thus can be used forterminals in any transmission mode, i.e., DCI format 0/1A is a formatcommonly usable in all terminals. In addition, when DCI format 0/1A isused, single-antenna transmission or transmit diversity is used as thedefault transmission mode.

A terminal receives DCI format 0/1A, and the DCI formats that aredependent on the downlink transmission mode. In addition, a terminalconfigured with an uplink spatial-multiplexing MIMO transmissionreceives DCI format 4 in addition to the abovementioned DCI formats.

The use of DCI format 0 and DCI format 4, which are control informationformats used for uplink data (PUSCH) assignment reporting, for reportingthe A-SRS trigger information has been discussed. The field forreporting an A-SRS trigger is added to DCI format 0 in addition to an RBreporting field, MCS reporting field, HARQ information reporting field,transmission power control command reporting field and terminal IDfield. In addition to the fields described above, DCI format 4 includesan MCS reporting field for the second transport block (data codeword) tobe spatially multiplexed, and precoding information for spatialmultiplexing.

The DCI described above is transmitted to a base station to a terminalvia a PDCCH. The base station in this case assigns a plurality ofterminals to a single subframe, so that the base station simultaneouslytransmits a plurality of PDCCHs using different resources. The basestation transmits the PDCCHs while including CRC bits which have beenmasked (or scrambled) using the terminal ID of the transmissiondestination in each of the PDCCHs in order to identify the terminal ofthe transmission destination of each of the PDCCHs. Each terminal thendetects the PDCCH intended for the terminal by blind-decoding the PDCCHsby demasking (or descrambling) CRC bits with the terminal ID of theterminal in the PDCCHs which may have been transmitted for the terminal.

CITATION LIST Non-Patent Literatures

NPL 1

-   3GPP TS 36.211 V8.7.0, “Physical Channels and Modulation (Release    8),” September 2008

NPL 2

-   3GPP TS 36.212 V8.7.0, “Multiplexing and channel coding (Release    8),” September 2008

NPL 3

-   3GPP TS 36.213 V8.7.0, “Physical layer procedures (Release 8),”    September 2008

NPL 4

-   3GPP TSG RAN WG1 meeting, R1-105632, “On Dynamic Aperiodic SRS    Transmission Timing,” October 2010

BRIEF SUMMARY Technical Problem

As described above, a terminal transmits an A-SRS in the first commonSRS subframe located at or after a k-th subframe (e.g., k=4) from thesubframe in which the terminal receives the trigger information. Morespecifically, let us suppose a case where the periodicity of common SRSsubframes is equal to Np subframes. In this case, when receiving anA-SRS trigger during a period from a subframe located Np+k+1 subframesbefore a certain common SRS subframe until a subframe located ksubframes before the common SRS subframe among Np subframes, theterminal uses the common SRS subframe to transmit an A-SRS. Stateddifferently, when requesting a terminal to transmit an A-SRS in subframen, which is a common SRS subframe, the base station notifies theterminal of the A-SRS transmission request by the A-SRS triggerinformation during a period from a subframe corresponding to subframen−(Np+k+1) until a subframe corresponding to subframe n−k among Npsubframes (hereinafter, referred to as “effective period”).

At least two kinds of DCI formats, which are DCI format 0 and DCI format4, can be used for an A-SRS transmission request. A base station canrequest each terminal to transmit an A-SRS of a different A-SRSconfiguration (e.g., A-SRS bandwidth, cyclic shift, Comb, the number ofantennas and/or the like) using each of the DCI formats.

Meanwhile, each terminal detects the DCI intended for the terminal byblind-decoding PDCCHs. For this reason, a terminal may erroneouslydetect DCI that is intended for a different terminal or DCI that has notbeen transmitted. Such erroneous detection of DCI is called a “falsealarm” or “false detection” and means that DCI intended for a differentterminal or a signal that has not been transmitted intentionally (i.e.,noise components) is erroneously detected as the DCI intended for theterminal. Each terminal makes CRC judgment for a plurality of PDCCHresource candidates after demasking the CRC part of each PDCCH using theterminal ID of the terminal (i.e., blind-decoding). During theblind-decoding, when the result of CRC is OK, the terminal detects theDCI as being intended for the terminal regardless of whether or not thebit sequence is actually correct or is intended for the terminal. Forexample, even when nothing is actually transmitted on the blind-decodingtarget PDCCH resource, the terminal blind-decodes noise components as asignal. In this case, a random bit sequence appears as the decodingresult, and the result of CRC becomes OK depending on the combination ofbits.

In addition, a base station may intentionally or unintentionally reportsa plurality of A-SRS transmission requests within an effective period.Accordingly, there is a possibility for a terminal to detect a pluralityof A-SRS transmission requests within an effective period for a certaincommon SRS subframe.

However, no studies have been carried out on the operation of terminalsupon detection of a plurality of A-SRS transmission requests. For thisreason, there is a concern that a base station may perform an erroneousreception quality measurement as a result of different understanding ofA-SRS configuration between the base station and the terminal. Inaddition, the terminal may unnecessarily interfere with a different cellin this case. In particular, A-SRS transmission performed by a terminalusing an SRS resource allocated to a different terminal affects not onlythe reception quality measurement of the terminal but also the receptionquality measurement of the different terminal, thus possibly degradingthe system throughput.

It is an object of the present invention to provide a communicationapparatus and an SRS transmission method each of which is capable ofpreventing degradation in the system throughput by reducing thepossibility of occurrence of a difference in understanding of thepresence or absence of SRS transmission or understanding of an SRSresource between the SRS transmission side and reception side.

Solution to Problem

A communication apparatus according to an aspect of the presentinvention includes: a detection section that detects control informationindicating whether or not to request transmission of a soundingreference signal (SRS); and a control section that controls thetransmission of the SRS based on the detected control information, inwhich, when detecting a plurality of pieces of the control informationwithin a predetermined period, the control section controls thetransmission of the SRS based on a piece of the control information thatis detected first.

A communication apparatus according to an aspect of the presentinvention includes: a detection section that detects control informationindicating whether or not to request transmission of a soundingreference signal (SRS); and a control section that controls thetransmission of the SRS based on the detected control information, inwhich, when detecting a plurality of pieces of the control informationwithin a predetermined period, the control section controls thetransmission of the SRS based on a piece of the control information thatis detected last.

A communication apparatus according to an aspect of the presentinvention includes: a detection section that detects control informationindicating whether or not to request transmission of a soundingreference signal (SRS); and a control section that controls thetransmission of the SRS based on the detected control information, inwhich, when detecting a plurality of pieces of the control informationwithin a predetermined period, the control section performs control insuch a way that the SRS is not transmitted.

A communication apparatus according to an aspect of the presentinvention includes: a detection section that detects control informationindicating whether or not to request transmission of a soundingreference signal (SRS); and a control section that controls thetransmission of the SRS based on the detected control information, inwhich, when detecting a plurality of different pieces of the controlinformation within a predetermined period, the control section performscontrol in such a way that the SRS is not transmitted.

A communication apparatus according to an aspect of the presentinvention includes: a detection section that detects control informationindicating whether or not to request transmission of a soundingreference signal (SRS); and a control section that controls thetransmission of the SRS based on the detected control information, inwhich, when detecting at least one piece of the control information thatrequests the transmission of the SRS and subsequently detecting a pieceof the control information that requests no transmission of the SRS,within a predetermined period, the control section performs control insuch a way that the SRS is not transmitted, and when further detecting,within the predetermined period, at least one piece of the controlinformation that is different from the at least one piece of the controlinformation and that requests the transmission of the SRS, the controlsection performs control in such a way that the SRS is transmitted,based on the piece of the control information that requests thetransmission of the SRS and that is detected last.

A communication apparatus according to an aspect of the presentinvention includes: a transmission section that transmits controlinformation indicating whether or not to request transmission of asounding reference signal (SRS) to a communication counterpart; and adetection section that detects the SRS transmitted at a predeterminedtiming based on the control information from the communicationcounterpart, in which the transmission section transmits only one pieceof the control information before the predetermined timing within apredetermined period.

A communication apparatus according to an aspect of the presentinvention includes: a transmission section that transmits controlinformation indicating whether or not to request transmission of asounding reference signal (SRS) to a communication counterpart; and adetection section that detects the SRS transmitted at a predeterminedtiming based on the control information from the communicationcounterpart, in which the transmission section transmits identicalpieces of the control information when transmitting a plurality ofpieces of the control information to the communication counterpartbefore the predetermined timing within a predetermined period.

An SRS transmission control method according to an aspect of the presentinvention includes: detecting control information indicating whether ornot to request transmission of a sounding reference signal (SRS); andcontrolling the transmission of the SRS based on the detected controlinformation and a predetermined rule when a plurality of pieces of thecontrol information is detected before a predetermined timing at whichthe SRS is transmitted within a predetermined period, or a plurality ofpieces of the control information is detected within the predeterminedperiod.

Advantageous Effects of Invention

According to the present invention, it is possible to provide acommunication apparatus and an SRS transmission method each of which iscapable of preventing degradation in the system throughput by reducingthe possibility of occurrence of a difference in understanding of thepresence or absence of SRS transmission or understanding of an SRSresource between the SRS transmission side and reception side.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a main configuration diagram of a base station according toEmbodiment 1 of the present invention;

FIG. 2 is a main configuration diagram of a terminal according toEmbodiment 1 of the present invention;

FIG. 3 is a block diagram of the base station according to Embodiment 1of the present invention;

FIG. 4 is a block diagram of the terminal according to Embodiment 1 ofthe present invention;

FIGS. 5A-5B are diagrams provided for describing the operation of a basestation and a terminal;

FIGS. 6A-6B are diagrams provided for describing the operation of a basestation and a terminal according to Embodiment 2 of the presentinvention;

FIGS. 7A-7B are diagrams provided for describing the operation of a basestation and a terminal according to Embodiment 3 of the presentinvention;

FIGS. 8A-8B are diagrams provided for describing the operation of a basestation and a terminal according to Embodiment 4 of the presentinvention; and

FIG. 9 is a diagram provided for describing the operation of a basestation and a terminal according to Embodiment 5 of the presentinvention.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings. Throughout the embodiments, thesame elements are assigned the same reference numerals and any duplicatedescription of the elements is omitted.

Embodiment 1 Overview of Communication System

A communication system according to Embodiment 1 of the presentinvention includes base station 100 and terminals 200. Base station 100is an LTE-A compliant base station and each terminal 200 is an LTE-Acompliant terminal.

FIG. 1 is a main configuration diagram of base station 100 according toEmbodiment 1 of the present invention. In base station 100, transmissionprocessing section 104 transmits control information indicating whetheror not to request a sounding reference signal (SRS) transmission toterminal 200, and reception processing section 108 detects an SRStransmitted from terminal 200 at a predetermined timing based on thecontrol information. In Embodiment 1, configuration section 101 controlsthe transmission of a transmission request, and only one piece ofcontrol information is transmitted within an effective period. Basestation 100 instructs terminal 200 to transmit an SRS, using an A-SRStransmission request transmitted from transmission processing section104.

FIG. 2 is a main configuration diagram of terminal 200 according toEmbodiment 1 of the present invention. In terminal 200, receptionprocessing section 203 detects control information indicating whether ornot transmission of a sounding reference signal (SRS) is requested, andtransmission signal forming section 207 transmits an A-SRS under thecontrol of transmission controlling section 206 based on the controlinformation. Transmission controlling section 206 determines whether ornot to perform SRS transmission on the basis of an “SRS transmissionexecution rule” and the reception condition of trigger information.

Hereinafter, a description will be provided on an FDD system in whichthe uplink and downlink are separated in the frequency domain.

Configuration of Base Station 100

FIG. 3 is a block diagram illustrating a configuration of base station100 according to Embodiment 1 of the present invention. In FIG. 3, basestation 100 includes configuration section 101, coding and modulationsections 102 and 103, transmission processing section 104, RFtransmitting section 105, antenna 106, RF receiving section 107,reception processing section 108, data receiving section 109, and SRSreceiving section 110.

Configuration section 101 generates “A-SRS transmission ruleconfiguration information” for configuring configuration target terminal200 with correspondence between control information formats (DCIformats) used for requesting A-SRS transmission and resources used byconfiguration target terminal 200 for the A-SRS transmission (i.e.,A-SRS resource). The A-SRS transmission rule configuration informationincludes identification information for a plurality of controlinformation formats (DCI formats) and information about the A-SRSresource corresponding to the identification information on each of thecontrol information formats. This A-SRS resource is a resource forconfiguration target terminal 200 to map an A-SRS as described above.The information about the A-SRS resource includes parameters forconfiguration target terminal 200 to transmit an A-SRS, such as thefrequency band (or RB position where the SRS band starts), frequencybandwidth (or the number of RBs), Cyclic Shift, transmission Comb, thenumber of antennas, the number of times the transmission is to beperformed, frequency hopping, and component carrier. More specifically,configuration target terminal 200 is configured with a combination ofthe identification information on each of a plurality of controlinformation formats (DCI formats) and the abovementioned parameterscorresponding to the identification information on a corresponding oneof the control information formats by the A-SRS transmission ruleconfiguration information. The A-SRS resources in accordance with thenumber of bits used as the trigger information for the DCI formats(i.e., the number of A-SRS resource candidates that can be reportedusing the trigger information) are associated respectively with bitstates that can be expressed by the number of bits. In the case of 1bit, for example, one bit state is used to report “no SRS transmissionrequest,” so that the number of resource types that can be reported islimited to only one. For this reason, the other bit state is associatedwith resource A. In the case of 2 bits, three resource types can bereported, so that three bit states are associated with three resourcesB, C, and D, respectively.

Configuration section 101 generates uplink assignment controlinformation or downlink assignment control information that includestrigger information requesting an A-SRS transmission for configurationtarget terminal 200 (hereinafter, simply referred to as “triggerinformation”).

The A-SRS transmission rule configuration information generated byconfiguration section 101 in the manner described above is subjected totransmission processing as control information of the RRC layer incoding and modulation section 102, transmission processing section 104,and RF transmitting section 105 and thereafter transmitted toconfiguration target terminal 200. The control information includingtrigger information for A-SRS transmission is subjected to transmissionprocessing in coding and modulation section 102, transmission processingsection 104, and RF transmitting section 105 as control information oflayers 1 and 2 and thereafter transmitted to configuration targetterminal 200. When the trigger information consists of 1 bit (e.g., DCIformat 0), the bit value “0” indicates an A-SRS transmission requestusing resource A, and the bit value “1” indicates no A-SRS transmissionrequest. When the trigger information consists of 2 bits (e.g., DCIformat 4), among four bit states, state 1 indicates no A-SRStransmission request, and states 2, 3 and 4 indicate an A-SRStransmission request using resources B, C, and D, respectively.Configuration section 101 configures resources A, B, C and D.

Configuration section 101 generates assignment control informationincluding trigger information, resource (RB) allocation information, andMCS information for one or more transport blocks (TBs) The assignmentcontrol information may be assignment control information on an uplinkresource for assigning uplink data (e.g., Physical Uplink Shared Channel(PUSCH)) or on a downlink resource for assigning downlink data (e.g.,Physical Downlink Shared Channel (PDSCH)). The assignment controlinformation for assigning uplink data includes DCI formats 0 and 4, andthe assignment control information for assigning downlink data includesDCI format 1A, 1, 1B, 1D, 2, 2A and/or the like.

Configuration section 101 generates control information includingtrigger information indicating a transmission request to a terminal inorder that the control information can be transmitted within theeffective period corresponding to the subframe in which the terminal iscaused to transmit an A-SRS. Embodiment 1 assumes that base station 100transmits control information including trigger information indicatingan SRS transmission request to one target terminal only once within asingle effective period.

Configuration section 101 transmits the A-SRS transmission ruleconfiguration information to configuration target terminal 200 viacoding and modulation section 102 and also outputs the information toreception processing section 108. Configuration section 101 transmitsthe assignment control information including trigger information toconfiguration target terminal 200 via coding and modulation section 102and also outputs the information to transmission processing section 104.Configuration section 101 outputs information indicating the format ofthe assignment control information including trigger information toreception processing section 108.

The configuration information is reported to terminal 200 from basestation 100 as higher layer information (i.e., via RRC signaling).Meanwhile, the assignment control information (including triggerinformation) is reported to terminal 200 from base station 100 usingPhysical Downlink Control Channel (PDCCH). Specifically, the reportingintervals for the configuration information are relatively long (i.e.,reported between relatively long intervals) while the reportingintervals for the assignment control information are relatively short(i.e., reported between relatively short intervals).

Coding and modulation section 102 encodes and modulates theconfiguration information and assignment control information receivedfrom configuration section 101 and outputs the resultant modulationsignal to transmission processing section 104.

Coding and modulation section 103 encodes and modulates data signals tobe received and outputs the obtained modulation signal to transmissionprocessing section 104.

Transmission processing section 104 maps the modulation signals to bereceived from coding and modulation sections 102 and 103 to the resourceindicated by the resource allocation information to be received fromconfiguration section 101, thereby forming a transmission signal. Forthis processing, when the transmission signal is an OFDM signal, themodulation signal is mapped to the resource indicated by the downlinkresource allocation information to be received from configurationsection 101, then transforms the signal into a time waveform by inversefast Fourier transform (IFFT) processing, and adds a cyclic prefix (CP)to the resultant signal, thereby forming an OFDM signal.

RF transmitting section 105 performs radio transmission processing (suchas up-conversion and digital to analog (D/A) conversion) on thetransmission signal to be received from transmission processing section104.

RF receiving section 107 performs radio reception processing (such asdown-conversion and analog to digital (A/D) conversion) and outputs theresultant received signal to reception processing section 108.

Reception processing section 108 identifies the resource to which theuplink data signal and ACK/NACK information are mapped, on the basis ofthe uplink resource allocation information received from configurationsection 101 and extracts signal components mapped to the identifiedresource from the received signal.

In addition, reception processing section 108 identifies the resource towhich an A-SRS is mapped, on the basis of the A-SRS transmission ruleconfiguration information, trigger information, and the information onthe DCI format used for the A-SRS transmission request received fromconfiguration section 101 and extracts signal components mapped to theidentified resource from the received signal. More specifically,reception processing section 108 receives the A-SRS on the identifiedresource in the first common SRS subframe located at or after a k-thsubframe (k=4 in this case) from the subframe in which the triggerinformation is transmitted.

When the received signal is a spatially multiplexed signal (i.e.,transmitted by a plurality of codewords (CWs)), reception processingsection 108 demultiplexes the received signals for each CW. In addition,when the received signal is an OFDM signal, reception processing section108 transforms the received signal into a time-domain signal byperforming inverse discrete Fourier transform (IDFT) processing on theextracted signal components.

The uplink data signal and ACK/NACK information extracted by receptionprocessing section 108 as described above are outputted to datareceiving section 109, and the A-SRS signal is outputted to SRSreceiving section 110.

Data receiving section 109 decodes the signal received from receptionprocessing section 108. Thus, the uplink data and ACK/NACK informationare obtained.

SRS receiving section 110 measures the reception quality of eachfrequency resource on the basis of the A-SRS signal received fromreception processing section 108 and outputs the reception qualityinformation. When a plurality of A-SRS signals transmitted fromdifferent terminals 200 are code-multiplexed using an orthogonalsequence and/or the like, SRS receiving section 110 performsdemultiplexing processing on the code-multiplexed A-SRS signals.

Configuration of Terminal 200

FIG. 4 is a block diagram illustrating a configuration of terminal 200according to Embodiment 1 of the present invention. Terminal 200 is anLTE-A compliant terminal in Embodiment 1.

In FIG. 4, terminal 200 includes antenna 201, RF receiving section 202,reception processing section 203, reference signal generating section204, data signal generating section 205, transmission controllingsection 206, transmission signal forming section 207, and RFtransmitting section 208.

RF receiving section 202 performs radio receiving processing (such asdown-conversion and analog to digital (A/D) conversion) on the radiosignal received via antenna 201 and outputs the resultant receivedsignal to reception processing section 203.

Reception processing section 203 extracts the configuration information,assignment control information, and data signal in the received signal.Reception processing section 203 outputs the configuration informationand assignment control information to transmission controlling section206. In addition, reception processing section 203 outputs the DCIformat identification information of the assignment control informationin which the trigger information has been included to transmissioncontrolling section 206. Reception processing section 203 performs errordetection processing on the extracted data signal and outputs ACK/NACKinformation in accordance with the result of error detection to datasignal generating section 205. Reception processing section 203 detectsDCI by blind-decoding and extracts the assignment control informationfrom the detected DCI.

Reference signal generating section 204 generates a reference signalupon reception of an instruction to generate a reference signal fromtransmission controlling section 206 and outputs the reference signal totransmission signal forming section 207.

Data signal generating section 205 takes the ACK/NACK information andtransmission data as input and generates a data signal by encoding andmodulating the ACK/NACK information and transmission data on the basisof MCS information to be received from transmission controlling section206. In the case of Non-MIMO transmission, a data signal is generated bysingle codeword (CW), while a data signal is generated by two codewordsin the case of MIMO. When the received signal is an OFDM signal, datasignal generating section 205 performs CP removal processing and FFTprocessing.

Transmission controlling section 206 determines whether or not toperform SRS transmission, on the basis of the “SRS transmissionexecution rule” and the reception condition of the trigger information.The “SRS transmission execution rule” in Embodiment 1 indicates that anSRS is transmitted in accordance with the assignment control informationthat includes the trigger information indicating a transmission requestand that is detected first within an effective period. Morespecifically, upon detection of trigger information indicating atransmission request for a common SRS subframe of subframe number n,once, transmission controlling section 206 disregards triggerinformation and assignment control information included in DCI even whendetecting DCI including trigger information indicating a transmissionrequest after the detection of the first trigger information within theeffective period in which an A-SRS of subframe number n can be requested(i.e., during a period from subframe n−(Np+k+1) until subframe n−k).

When determining to perform SRS transmission, transmission controllingsection 206 configures a resource for terminal 200 to map an A-SRSsignal. Specifically, transmission controlling section 206 identifiesthe resource on the basis of the configuration information (A-SRStransmission rule configuration information), and DCI formatidentification information of the assignment control information inwhich the trigger information has been included, which are to bereceived from reception processing section 203. In addition, whenmultiple bits are included as trigger information, SRS resourcereporting information included in the trigger information is used foridentifying the resource.

Transmission controlling section 206 configures the first common SRSsubframe located at or after a k-th subframe from the subframe in whichthe assignment control information including the trigger information istransmitted, to be the transmission subframe for an A-SRS. Upon receiptof the trigger information, transmission controlling section 206 outputsan instruction to generate a reference signal to reference signalgenerating section 204 and outputs information about the identified SRSresource to transmission signal forming section 207.

Transmission controlling section 206 identifies a “data mappingresource” to which the data signal is mapped, on the basis of theassignment control information to be received from reception processingsection 203 and outputs information about the data mapping resource(hereinafter, referred to as “data mapping resource information”) totransmission signal forming section 207 and also outputs MCS informationincluded in the assignment control information to data signal generatingsection 205.

Transmission signal forming section 207 maps the A-SRS signal to bereceived from reference signal generating section 204 to the SRS mappingresource. Transmission signal forming section 207 maps the data signalto be received from data signal generating section 205 to the datamapping resource indicated by the data mapping resource information. Thetransmission signal is formed in the manner described above. In the caseof Non-MIMO transmission, a single codeword data signal is assigned to asingle layer, while a two codeword data signal is assigned to aplurality of layers in the case of MIMO transmission. When thetransmission signal is an OFDM signal, transmission signal formingsection 207 performs discrete Fourier transform (DFT) processing on thedata signal and thereafter maps the processed data signal to the datamapping resource. Meanwhile, a CP is added to the formed transmissionsignal.

RF transmitting section 208 performs radio transmission processing (suchas up-conversion and digital to analog (D/A) conversion) on thetransmission signal formed by transmission signal forming section 207and transmits the processed signal via antenna 201.

Operation of Base Station 100 and Terminal 200

A description will be provided with reference to FIG. 5, regarding theoperation of base station 100 and terminal 200 configured in the mannerdescribed above. This description assumes that the assignment controlinformation of DCI format 0 and DCI format 4 includes triggerinformation. FIG. 5 describe the processing related to the uplink dataassignment and A-SRS transmission request in base station 100 and to thedata transmission and A-SRS transmission in terminal 200. The dataassignment reporting and data transmission are performed on a persubframe basis.

As illustrated in FIG. 5A, base station 100 transmits at most one pieceof DCI that includes an A-SRS transmission request to each terminal 200within an effective period corresponding to one common SRS subframe.

Meanwhile, transmission controlling section 206 determines whether ornot to perform SRS transmission on the basis of the “SRS transmissionexecution rule” and the reception condition of the trigger informationin terminal 200. Specifically, the “SRS transmission execution rule” inEmbodiment 1 indicates that an SRS is transmitted in accordance with theassignment control information which includes trigger informationindicating a transmission request and which is detected first within aneffective period. More specifically, even if DCI that includes an A-SRStransmission request is detected after detection of first DCI thatincludes an A-SRS transmission request within a single effective period,the DCI detected after the first DCI is disregarded. Stated differently,when a plurality of pieces of DCI that include trigger informationindicating an A-SRS transmission request are detected within a singleeffective period, no A-SRS is transmitted on the SRS resource requestedby a piece of DCI that includes an A-SRS transmission request and thatis detected after the first DCI, but an A-SRS is transmitted on the SRSresource requested by the first piece of DCI that includes an A-SRStransmission request and that is detected first (see, FIG. 5B).

The control of SRS transmission according to the “SRS transmissionexecution rule” described above makes it possible to reduce theprobability of terminal 200 erroneously detecting DCI or the probabilityof terminal 200 transmitting an A-SRS using an SRS resource differentfrom an SRS resource requested by base station 100, due to erroneousdetection of A-SRS trigger information. In this situation, when terminal200 erroneously detects DCI that includes an A-SRS transmission requestbefore base station 100 transmits DCI that includes an A-SRStransmission request, terminal 200 wrongly transmits an A-SRS. However,when base station 100 reports an A-SRS transmission request using anearliest possible subframe within an effective period, the probabilityof terminal 200 erroneously detecting DCI before base station 100reports an A-SRS transmission request within the effective period can bereduced. In addition, this configuration provides an advantage in thatthe processing of terminal 200 becomes very simple because terminal 200needs to prepare for A-SRS transmission only according to the firsttrigger information.

When detecting a different piece of DCI that includes an A-SRStransmission request after detecting a first piece of DCI that includesan A-SRS transmission request within a single effective period, terminal200 may treat data assignment according to the different piece of DCI(i.e., information about data assignment such as data RBs, MCS,transmission power control, and/or the like) as valid information anddisregards only the A-SRS trigger information included in the differentpiece of DCI. Specifically, the assignment reporting information on thedata assignment and the A-SRS trigger information in a piece of DCI maybe treated, independently. In this case, no A-SRS is transmitted usingthe SRS resource indicated by the DCI that includes an A-SRStransmission request and that is detected after the first DCI, but anA-SRS is transmitted using the SRS resource requested by the DCI thatincludes an A-SRS transmission request and that is detected first withina single effective period, while uplink data (PUSCH) is transmittedaccording to the data assignment information indicated by the DCI thatincludes an A-SRS transmission request and that is detected after thefirst DCI. Stated differently, the information other than the A-SRStrigger information in the information included in the assignmentreporting information in DCI (i.e., RB allocation, MCS reportinginformation, and/or the like) is treated as valid regardless of thestate of any A-SRS trigger information detected before the DCI, and theA-SRS trigger information detected before the DCI is determined to bevalid or invalid depending on the presence or absence of the A-SRStrigger information detected before the DCI.

The A-SRS and A-SRS trigger information are newly introduced inLTE-Advanced. Meanwhile the assignment reporting information other thanA-SRS trigger information in DCI is already defined in LTE.Specifically, the processing circuit configured to perform the operationrelating to the assignment reporting information in DCI has been alreadyimplemented in LTE base stations and LTE terminals before LTE-Advanced.Thus, determining validity or invalidity of the assignment reportinginformation and A-SRS trigger information in a piece of DCIindependently makes it possible to continue using the processing circuitthat has been already implemented in LTE without any modification asdescribed above. Accordingly, the man-hours required for implementationcan be reduced since it requires only addition of the processing partcorresponding to the A-SRS related part.

Embodiment 2

In Embodiment 2, the “SRS transmission execution rule” indicates that anA-SRS is transmitted according to the assignment control informationthat includes trigger information indicating a transmission request andthat is detected last within an effective period. The base station andterminal according to Embodiment 2 are similar to base station 100 andterminal 200 according to Embodiment 1 in their basic configurations, sothat Embodiment 2 will be described with reference to FIGS. 3 and 4.

In base station 100 according to Embodiment 2, configuration section 101generates DCI that includes trigger information indicating an A-SRStransmission request in one or more subframes within an effective periodfor each terminal 200. Specifically, base station 100 according toEmbodiment 2 is capable of reallocating an A-SRS resource to eachterminal 200 within an effective period corresponding to a certaincommon SRS subframe in accordance with the A-SRS assignment state for aplurality of terminals 200 in the common SRS subframe.

In terminal 200 according to Embodiment 2, transmission controllingsection 206 determines whether or not to perform SRS transmission on thebasis of the “SRS transmission execution rule” and the receptioncondition of trigger information. The “SRS transmission execution rule”in Embodiment 2 indicates that an A-SRS is transmitted according to theassignment control information that includes trigger informationindicating a transmission request and that is detected last within aneffective period. Specifically, when detecting a first piece of DCI thatincludes trigger information indicating a transmission request in aneffective period corresponding to a common SRS subframe of subframenumber n and detecting a different piece of DCI that includes triggerinformation indicating a transmission request in a different subframetransmitted after the subframe in which the first piece of DCI isdetected (i.e., during a period from the initial detection timing untila subframe corresponding to subframe n−k) in the same period,transmission controlling section 206 overwrites the information aboutthe SRS resource indicated by the previous trigger information with theinformation about the SRS resource indicated by the trigger informationin the piece of DCI detected this time. Accordingly, transmissioncontrolling section 206 can hold the latest information about the SRSresource in each effective period. Transmission controlling section 206outputs an instruction to generate a reference signal to referencesignal generating section 204 and also outputs the updated informationabout the SRS resource to transmission signal forming section 207 inaccordance with the result of overwriting the information.

The operation of base station 100 and terminal 200 according toEmbodiment 2, which are configured in the manner described above, willbe described with reference to FIGS. 6. In FIGS. 6A and 6B, base station100 transmits DCI that includes trigger information indicating an A-SRStransmission request in one subframe and also transmits DCI thatincludes trigger information indicating an A-SRS transmission request ina subsequent subframe in a single effective period.

In FIG. 6A, terminal 200 correctly detects all pieces of DCI andreceives trigger information indicating an A-SRS transmission request intwo subframes. Terminal 200 then transmits an A-SRS according to the“SRS transmission execution rule” and the assignment control informationthat includes trigger information indicating a transmission request andthat is detected last within the effective period.

In FIG. 6B, terminal 200 fails to detect DCI that includes triggerinformation indicating an A-SRS transmission request and that istransmitted first within a single effective period. Specifically,terminal 200 fails to recognize that the DCI is intended for terminal200 because the result of error detection does not indicate OK as aresult of the presence of a bit error. In this effective period,however, the DCI that indicates trigger information indicating an A-SRStransmission request and that is transmitted after the DCI that hasresulted in detection failure is correctly detected. Thus, terminal 200transmits an A-SRS according to the A-SRS trigger information includedin the correctly detected DCI.

As described above, base station 100 transmits DCI that includes triggerinformation indicating an A-SRS transmission request in a plurality ofsubframes within a single effective period, while the transmission ofSRS is controlled according to the “SRS transmission execution rule.”Thus, terminal 200 can correctly transmit an A-SRS even when failing todetect DCI transmitted during an early stage of the effective period, aslong as terminal 200 can detect DCI transmitted after the earlier DCI.

As a result, base station 100 can measure appropriate channel quality.

In addition, base station 100 can change an SRS resource by transmittinga plurality of pieces of trigger information within an effective period.Specifically, base station 100 can request A-SRS transmission to beperformed using an SRS resource different from the SRS resource reportedby first trigger information, by transmitting second trigger informationindicating the different SRS resource, after the first triggerinformation. For example, when base station 100 needs a certain terminal200 with higher priority to perform A-SRS transmission using an SRSresource (e.g., resource A) after transmitting an A-SRS transmissionrequest with the same SRS resource (resource A) to a different terminal200, base station 100 transmits A-SRS trigger information indicating adifferent SRS resource (e.g., resource B) for overwriting the previouslyindicated SRS resource. The different SRS resource may be indicated byusing a different DCI format or using the same DCI format but reportinga different state of the A-SRS trigger information. As described above,base station 100 can reallocate an SRS resource to terminal 200 byoverwriting an SRS resource that has been reported to terminal 200 witha different resource. Accordingly, it is possible to prevent a collisionbetween SRS resources allocated to a plurality of terminals 200.

As described above, when terminal 200 erroneously detects DCI, an A-SRSmay be transmitted using a wrong SRS resource. However, when terminal200 correctly receives DCI that includes trigger information indicatingan A-SRS transmission request and that is transmitted in the lastsubframe within the effective period, terminal 200 does not transmit anA-SRS using a wrong SRS resource. Alternatively, reporting DCI thatincludes trigger information indicating an A-SRS transmission requestfrom base station 100, using a latest possible subframe in the effectiveperiod makes it possible to reduce the probability of terminal 200erroneously detecting DCI or the probability of terminal 200transmitting an A-SRS using an SRS resource different from the SRSresource required by base station 100, due to erroneous detection ofA-SRS trigger information.

Embodiment 3

In Embodiment 3, the “SRS transmission execution rule” indicates thatupon detection of a plurality of pieces of DCI each of which includestrigger information indicating an A-SRS transmission request within aneffective period, no A-SRS is transmitted in any common SRS subframecorresponding to the effective period. The base station and terminalaccording to Embodiment 3 are similar to base station 100 and terminal200 according to Embodiment 1 in their basic configurations, so thatEmbodiment 3 will be described with reference to FIGS. 3 and 4.

In base station 100 according to Embodiment 3, configuration section 101generates DCI that includes trigger information indicating an A-SRStransmission request in one or two subframes within an effective periodfor each terminal 200. The number of times this trigger information isgenerated is controlled in accordance with the A-SRS assignment statefor a plurality of terminals 200 in a certain common SRS subframe. Forexample, when base station 100 needs a certain terminal 200 with higherpriority to perform A-SRS transmission using an SRS resource (e.g.,resource A) after transmitting an A-SRS transmission request using thesame SRS resource (resource A) to a different terminal 200, base station100 transmits A-SRS trigger information indicating an A-SRS transmissionrequest again within the same effective period to cancel the previouslymade A-SRS transmission request.

In terminal 200 according to Embodiment 3, transmission controllingsection 206 determines whether or not to perform SRS transmission on thebasis of the “SRS transmission execution rule” and the receptioncondition of the trigger information. In Embodiment 3, the “SRStransmission execution rule” indicates that upon detection of aplurality of pieces of DCI each of which includes trigger informationindicating an A-SRS transmission request within an effective period, noA-SRS is transmitted in any common SRS subframe corresponding to theeffective period. Specifically, when transmission controlling section206 detects first piece of DCI that includes trigger informationindicating a transmission request in an effective period correspondingto a common SRS subframe of subframe number n and detecting a differentpiece of DCI that includes trigger information indicating a transmissionrequest in a different subframe transmitted after the subframe in whichthe first piece of DCI is detected (i.e., during a period from theinitial detection timing until a subframe corresponding to subframe n−k)in the same period, transmission controlling section 206 determines theA-SRS trigger information to be invalid and also invalidates (i.e.,cancels) the A-SRS trigger information that is detected before the A-SRStrigger information. In this case, transmission controlling section 206outputs an instruction to cancel the instruction to generate a referencesignal to reference signal generating section 204.

The operation of base station 100 and terminal 200 according toEmbodiment 3 configured in the manner described above will be describedwith reference to FIGS. 7. In FIGS. 7A and 7B, base station 100transmits DCI that includes trigger information indicating an A-SRStransmission request in a single subframe and transmits DCI thatincludes trigger information indicating no A-SRS transmission request inuplink data assignment reporting in subframes other than the singlesubframe.

In FIG. 7A, terminal 200 correctly receives all pieces of DCI intendedfor terminal 200 without erroneous detection of DCI within the effectiveperiod. In addition, terminal 200 transmits an A-SRS using an SRSresource according to the A-SRS trigger information indicating atransmission request and detected in the single subframe within theeffective period.

Meanwhile, in FIG. 7B, although base station 100 transmits DCI intendedfor terminal 200 in two subframes within a single effective period,terminal 200 detects DCI that includes trigger information indicating anA-SRS transmission request in a different subframe (i.e., erroneousdetection). Since terminal 200 detects two pieces of DCI each of whichincludes trigger information indicating an A-SRS transmission requestwithin the single effective period, terminal 200 transmits no A-SRSaccording to the “SRS transmission execution rule.”

The SRS transmission control according to the “SRS transmissionexecution rule” as described above can reduce the probability ofterminal 200 erroneously transmitting an A-SRS due to erroneousdetection. Meanwhile, when terminal 200 erroneously detects DCI thatincludes trigger information indicating an A-SRS transmission requestalthough base station 100 has not made an A-SRS transmission requesteven once during a certain effective period, terminal 100 erroneouslytransmits an A-SRS. However, when base station 100 makes an A-SRStransmission request even once during a single effective period,erroneous transmission of an A-SRS due to erroneous detection byterminal 200 can be prevented. In addition, erroneous transmission ofdata due to erroneous detection of the second DCI indicating the secondA-SRS transmission request can be prevented.

The control of SRS transmission according to the “SRS transmissionexecution rule” described above allows base station 100 to cancel anA-SRS transmission request that has been reported to a certain terminal200, by intentionally transmitting a plurality of transmission requests.Thus, the resource that has become available because of the cancellationcan be allocated to different terminal 200 with higher priority.Accordingly, A-SRS resource allocation to terminals 200 according topriority is made possible, which in turn makes it possible to reduce theamount of delay in acquiring A-SRS reception quality information forterminal 200 with higher priority. As a result, the data transmissiondelay to terminal 200 with higher priority can be reduced.

When detecting a different piece of DCI that includes an A-SRStransmission request after detecting a first piece of DCI that includesan A-SRS transmission request in a single effective period, terminal 200may treat data assignment according to the different piece of DCI asvalid and treat only trigger information included in the different pieceof DCI as invalid, or may treat both of the data assignment and triggerinformation as invalid. In the former case, when canceling an A-SRStransmission request that has been reported to certain terminal 200,base station 100 can perform new data assignment while canceling onlythe A-SRS transmission request. The latter case is effective when thereis no data to be newly assigned, because base station 100 can cancelonly an A-SRS transmission request without involving data assignment,and terminal 200 transmits no wasteful data. In addition, the former andthe latter can be switched according to additional control informationtransmitted from base station 100 to terminal 200.

Embodiment 4

In Embodiment 4, the “SRS transmission execution rule” indicates that noA-SRS is transmitted upon detection of even one piece of DCI thatincludes trigger information indicating no A-SRS transmission afterdetection of a first piece of DCI that includes the trigger informationindicating an A-SRS transmission request, or upon detection of a pieceof DCI indicating an SRS resource different from the SRS resourceindicated by the first piece of DCI within an effective period. InEmbodiment 4, when transmitting the first piece of DCI that includestrigger information indicating an A-SRS transmission request, andthereafter reporting data assignment using a different piece of DCI ofthe same DCI format as that of the first piece of DCI, the base stationincludes, in the different piece of DCI, trigger information indicatingthe same SRS resource as the SRS resource indicated by the triggerinformation in the first piece of DCI. Stated differently, an assumptionis made that base station 100 repeatedly transmits a piece of DCI thatincludes the trigger information indicating the same SRS resource in asingle effective period. The base station and terminal according toEmbodiment 4 are similar to base station 100 and terminal 200 accordingto Embodiment 1 in their basic configurations, so that Embodiment 4 willbe described with reference to FIGS. 3 and 4.

In base station 100 according to Embodiment 4, when generating a firstpiece of DCI that includes trigger information indicating an A-SRStransmission request to terminal 200 in one subframe in a singleeffective period, configuration section 101 includes trigger informationindicating the A-SRS transmission request in a different piece of DCI tobe transmitted thereafter within the same effective period if a“predetermined condition” is satisfied. The “predetermined condition” isthat the DCI format of a different piece of DCI is the same as that ofthe first piece of DCI. The SRS resource indicated by the triggerinformation included in the different piece of DCI is also matched withthe SRS resource indicated by the trigger information included in thefirst piece of DCI. Stated differently, base station 100 according toEmbodiment 4 repeatedly transmits a piece of DCI that includes thetrigger information indicating the A-SRS transmission request within asingle effective period as long as the “predetermined condition” issatisfied, basically.

In terminal 200 according Embodiment 4, transmission controlling section206 determines whether or not to perform SRS transmission on the basisof the “SRS transmission execution rule” and the reception condition oftrigger information. In Embodiment 4, the “SRS transmission executionrule” indicates that upon detection of a piece of DCI that includestrigger information indicating no A-SRS transmission request even onceafter detection of a first piece of DCI that includes triggerinformation indicating an A-SRS transmission request, or upon detectionof a piece of DCI that indicates an SRS resource different from the SRSresource indicated by the first piece of DCI within an effective period,no A-SRS is transmitted in any common SRS subframe corresponding to theeffective period. Specifically, when detecting a piece of DCI thatincludes trigger information indicating a transmission request first inan effective period corresponding to the common SRS subframe of subframen and thereafter detecting a piece of DCI that includes triggerinformation indicating no A-SRS transmission request in a differentsubframe after the subframe in which the first piece of DCI is detected(i.e., period from the initial detection timing to the subframecorresponding to subframe n−k) even once in the effective period,transmission controlling section 206 determines that there is no A-SRStransmission. In addition, when detecting a piece of DCI that includestrigger information indicating an A-SRS transmission request using anSRS resource different from the SRS resource indicated by the firstpiece of DCI, transmission controlling section 206 also determines thatthere is no A-SRS transmission. In this case, transmission controllingsection 206 outputs an instruction to cancel the instruction to generatea reference signal to reference signal generating section 204.

The operation of base station 100 and terminal 200 according toEmbodiment 4 configured in the manner described above will be describedwith reference to FIG. 8.

In FIG. 8A, base station 100 transmits a piece of DCI that includestrigger information indicating the presence of an A-SRS transmissionrequest in a single subframe and repeatedly transmits a piece of DCIthat includes trigger information indicating the presence of an A-SRStransmission request after the single subframe in a single effectiveperiod. In FIG. 8A, terminal 200 correctly detects all pieces of DCI.Terminal 200 transmits an A-SRS according to the “SRS transmissionexecution rule” under the detection state of the pieces of DCI.

On the other hand, FIG. 8B illustrates a case where base station 100makes no A-SRS transmission request to terminal 200 within a certaineffective period. In FIG. 8B, base station 100 transmits DCI thatincludes trigger information indicating no A-SRS transmission request intwo subframes within the effective period. Meanwhile, terminal 200detects two pieces of DCI actually transmitted from base station 100after erroneously detecting a piece of DCI that includes triggerinformation indicating an A-SRS transmission request. Terminal 200transmits no A-SRS under this detection state of the pieces of DCIaccording to the “SRS transmission execution rule.”

In addition, although the operation is not illustrated in the drawings,when erroneously detecting that the third piece of DCI transmitted frombase station 100 and thus determining that the third piece of DCI is DCIthat includes A-SRS trigger information indicating an A-SRS transmissionrequest using resource B in FIG. 8A, terminal 200 performs no A-SRStransmission because the indicated resource is a resource different fromthe resource indicated by the previously detected A-SRS triggerinformation.

The control of SRS transmission according to the DCI transmission ruleof base station 100 and the “SRS transmission execution rule” ofterminal 200 in the manner described above, even when erroneouslydetecting DCI that includes trigger information indicating A-SRStransmission, terminal 200 can determine that terminal 200 haserroneously detected the DCI because of trigger information included inthe DCI to be received by terminal 200 after the erroneous detection ofDCI. Accordingly, the probability of terminal 200 erroneously detectingDCI, or the probability of terminal 200 transmitting an A-SRS using anSRS resource different from the SRS resource required by base station100 due to erroneous detection of A-SRS trigger information by terminal200 can be reduced.

As long as base station 100 satisfies the abovementioned “predeterminedcondition,” repeatedly transmitting DCI that includes triggerinformation indicating an A-SRS transmission request in a singleeffective period allows an A-SRS to be correctly transmitted even whenterminal 200 erroneously detects DCI that includes trigger informationindicating an A-SRS transmission request. As a result, base station 100can measure appropriate channel quality.

In addition, the control of SRS transmission according to the “SRStransmission execution rule” as described above allows base station 100to cancel an A-SRS transmission request that has been reported to acertain terminal 200. For example, when base station 100 makes an A-SRStransmission request with a certain SRS resource (e.g., resource A) tofirst terminal 200 first but needs second terminal 200 with higherpriority to transmit an A-SRS using the same resource (resource A), basestation 100 transmits assignment control information that is of the sameDCI format as that used for the previous A-SRS transmission request andthat includes trigger information indicating no A-SRS transmissionrequest to terminal 200, in order to cancel the A-SRS transmissionrequest that has been transmitted to first terminal 200. In this case,according to the “SRS transmission execution rule,” first terminal 200transmits no A-SRS. The resource thus made available in this case can beallocated to different terminal 200 with higher priority. Thus, A-SRSresource allocation to terminals 200 according to priority is madepossible, and the amount of delay in acquiring A-SRS reception qualityinformation for terminal 200 with higher priority can be reduced. As aresult, the data transmission delay to terminal 200 with higher prioritycan be reduced.

The control of SRS transmission according to the “transmission executionrule” described above controls terminals in a way that prevents theterminals from transmitting any A-SRS when the terminals detect DCI thatincludes trigger information indicating an A-SRS transmission requestand thereafter detects DCI that indicates an SRS resource different fromthe SRS resource indicated by the first DCI within an effective period.Accordingly, it is possible to reduce the probability of a terminaltransmitting an A-SRS using a wrong resource due to erroneous detectionof DCI that indicates A-SRS trigger information indicating a resourcedifferent from a resource actually indicated by the base station withinan effective period, thus, reducing the probability of the terminalunnecessarily interfering with a different terminal or a different cell.

It should be noted that, it is possible to use only any one of the two“SRS transmission execution rules” in this embodiment. For example, itis possible to use only the “SRS transmission execution rule” thatindicates that no A-SRS is transmitted upon detection of even one pieceof DCI that includes trigger information indicating no A-SRStransmission request after detection of the first DCI that includestrigger information indicating an A-SRS transmission request, or to useonly the “SRS transmission execution rule” that indicates no A-SRS istransmitted when a terminal receives DCI that includes triggerinformation indicating an A-SRS transmission request first andthereafter detects DCI that indicates an SRS resource different from theSRS resource indicated by the first DCI. In this case, the DCI thatindicates a different SRS resource is DCI that includes triggerinformation indicating an A-SRS configuration including a different SRSresource.

Embodiment 5

In Embodiment 5, the “SRS transmission execution rule” indicates that noA-SRS is transmitted upon detection of first DCI that includes triggerinformation indicating an A-SRS transmission request and detection ofeven one piece of different DCI that includes trigger informationindicating no A-SRS transmission request within an effective period, asin Embodiment 4. However, Embodiment 5 is different from Embodiment 4 inthat the “SRS transmission execution rule” indicates that even upondetection of a different piece of DCI that indicates an SRS resourcedifferent from the SRS resource indicated by the first piece of DCI, anA-SRS is transmitted according to the trigger information included inthe different piece of DCI when the DCI format of the different piece ofDCI is different from the DCI format of the first piece of DCI. As inEmbodiment 4, when transmitting a first piece of DCI that includestrigger information indicating an A-SRS transmission request andthereafter reporting data assignment by a piece of DCI different fromthe first DCI but of the same DCI format as that of the first piece ofDCI in an effective period, the base station includes, in the differentpiece of DCI, trigger information indicating the same SRS resource asthe SRS resource indicated by the trigger information in the first pieceof DCI in Embodiment 5. Specifically, an assumption is made that basestation 100 repeatedly transmits DCI that includes trigger informationindicating the same SRS resource within a single effective period. Thebase station and terminal according to Embodiment 5 are similar to basestation 100 and terminal 200 according to Embodiment 1 in their basicconfigurations, so that Embodiment 5 will be described with reference toFIGS. 3 and 4.

In base station 100 according to Embodiment 5, when generating a firstpiece of DCI that includes trigger information indicating an A-SRStransmission request for terminal 200 in one subframe within aneffective period, configuration section 101 includes trigger informationindicating an A-SRS transmission request in a different piece of DCI tobe transmitted thereafter within the effective period when a“predetermined condition” is satisfied. The “predetermined condition” isthat the DCI format of the different piece of DCI is the same as that ofthe first piece of DCI. In addition, the SRS resource indicated by thetrigger information included in the different piece of DCI is alsomatched with the SRS resource indicated by the trigger informationincluded in the first piece of DCI. Specifically, base station 100according to Embodiment 5 repeatedly transmits DCI that includes triggerinformation indicating an A-SRS transmission request within a singleeffective period as long as the “predetermined condition” is satisfied,basically.

However, when reallocating an A-SRS resource used in a previoustransmission request to each terminal 200, in accordance with the A-SRSassignment state for terminals 200, configuration section 101 changesthe DCI format of the assignment control information to another andincludes trigger information indicating an A-SRS transmission request inthe assignment control information.

In terminal 200 according to Embodiment 5, transmission controllingsection 206 determines whether or not to perform SRS transmission on thebasis of the “SRS transmission execution rule” and the receptioncondition of trigger information. The “SRS transmission execution rule”in Embodiment 5 indicates that upon detection of a piece of DCI thatincludes trigger information indicating no A-SRS transmission requesteven once after detection of a piece of DCI that includes triggerinformation indicating an A-SRS transmission request within an effectiveperiod, no A-SRS is transmitted in any common SRS subframe correspondingto the effective period. Specifically, when detecting a piece of DCIthat includes trigger information indicating an A-SRS transmissionrequest once in an effective period corresponding to a common SRSsubframe of subframe number n and detecting a piece of DCI that includestrigger information indicating no A-SRS transmission request in adifferent subframe transmitted after the common SRS subframe (i.e.,during a period from the initial detection timing until a subframecorresponding to subframe n−k) in the same period, transmissioncontrolling section 206 determines that no A-SRS transmission isrequired. In this case, transmission controlling section 206 outputs aninstruction to cancel the instruction to generate a reference signal toreference signal generating section 204.

In addition, the “SRS transmission execution rule” in Embodiment 5includes a rule indicating that even upon detection of a different pieceof DCI indicating an SRS resource different from the SRS resourceindicated by the first piece of DCI that includes trigger informationindicating an A-SRS transmission request, an A-SRS is transmittedaccording to the trigger information included in the different piece ofDCI when the DCI format of the different piece of DCI is different fromthe DCI format of the first piece of DCI. Specifically, transmissioncontrolling section 206 overwrites the information about the SRSresource indicated by the trigger information included in the piece ofDCI detected immediately before the different piece of DCI with theinformation about the SRS resource indicated by the trigger informationincluded in the different piece of DCI. In this case, transmissioncontrolling section 206 outputs an instruction to generate a referencesignal to reference signal generating section 204 and also outputs theinformation about the updated SRS resource to transmission signalforming section 207.

The operation of base station 100 and terminal 200 according toEmbodiment 5 configured in the manner described above will be describedwith reference to FIG. 9. In FIG. 9, base station 100 transmits a pieceof DCI that includes trigger information indicating an A-SRStransmission request in one subframe and repeatedly transmits a piece ofDCI that includes trigger information indicating an A-SRS transmissionrequest in subsequent subframes in a single effective period. In thiscase, an A-SRS transmission request for a plurality of antennas is madeby DCI format 4 (i.e., MIMO transmission assignment information).Moreover, an A-SRS transmission request for a plurality of antennas ismade using resource A.

In addition, in FIG. 9, base station 100 transmits a piece of DCI formedby including trigger information indicating an A-SRS transmissionrequest in assignment control information of DCI format 0 afterrepeatedly transmitting a piece of DCI formed by including triggerinformation indicating an A-SRS transmission request in assignmentcontrol information of DCI format 4. This is because terminal 200 iscaused to transmit an A-SRS using a resource different from a previouslyrequested SRS resource (e.g., single antenna transmission using resourceB) due to a change in the propagation path conditions of terminal 200(such as a case where data transmission becomes difficult using spatialmultiplexing transmission using a plurality of antennas due todegradation of the quality of propagation path, for example).

Meanwhile, terminal 200 detects an A-SRS transmission request using DCIformat 0 after detecting an A-SRS transmission request using DCI format4. According to the “SRS transmission execution rule,” terminal 200transmits an A-SRS according to the A-SRS trigger information of DCIdetected in a later subframe (i.e., trigger information of DCI format 0)under the detection state of DCI.

According to the DCI transmission rule by base station 100 and the “SRStransmission execution rule” by terminal 200, base station 100 canchange the SRS resource to another by transmitting a plurality of piecesof trigger information of different DCI formats within the effectiveperiod. Thus, base station 100 can cause each terminal 200 to transmitan A-SRS using an SRS resource corresponding to a change in thepropagation path conditions of each terminal 200. In addition, it ispossible to prevent a collision between SRS resources allocated to aplurality of terminals 200.

Moreover, as in Embodiment 4, as long as base station 100 satisfies theabovementioned “predetermined condition,” repeatedly transmitting apiece of DCI that includes trigger information indicating an A-SRStransmission request within a single effective period allows an A-SRS tobe correctly transmitted even when terminal 200 fails to detect a pieceof DCI that includes trigger information indicating an A-SRStransmission request. As a result, base station 100 can measureappropriate channel quality.

As in Embodiment 4, the control of SRS transmission according to the“SRS transmission execution rule” described above allows base station100 to cancel an A-SRS transmission request that has been reported tocertain terminal 200.

The description provided above assumes that base station 100 repeatedlytransmits a piece of DCI that includes trigger information indicating anA-SRS transmission request within a single effective period as long asthe “predetermined condition” is satisfied basically, as in Embodiment4. Meanwhile, when a first piece of DCI that includes triggerinformation indicating an A-SRS transmission request to terminal 200 isgenerated in one subframe with in an effective period, triggerinformation indicating an A-SRS transmission request may not be includedin a different piece of DCI that is to be transmitted after the firstpiece of DCI during the effective period and that is of the same formatas that of the first piece of DCI as in Embodiment 3. In this case,terminal 200 performs no A-SRS transmission when detecting the differentpiece of DCI that includes trigger information indicating an A-SRStransmission request and that is of the same DCI format as that of thefirst piece of DCI. However, when further detecting a piece of DCI thatincludes trigger information indicating an A-SRS transmission requestand that is of a DCI format different from that of the first piece ofDCI, terminal 200 transmits an A-SRS using an SRS resource according tothe A-SRS trigger information included in the piece of DCI of thedifferent DCI format.

An SRS resource may be changed to another only upon detection of DCIthat includes trigger information indicating an A-SRS transmissionrequest and that is of DCI format 0 after detection of DCI that includestrigger information indicating an A-SRS transmission request and that isof a DCI format other than DCI format 0. Specifically, overwriting theSRS resource corresponding to a previous A-SRS transmission request(including the number of antennas) by base station 100 is allowed onlyfor A-SRS transmission requests for single antenna transmission. A-SRStransmission requests for single antenna or A-SRS transmission requestsusing DCI format 0 are made when terminal 200 in the transmission modeusing multiple antenna transmission is caused to fall back to singleantenna transmission. In particular, such an A-SRS transmission requestis made when the propagation path conditions of terminal 200 aredegraded. Terminal 200 in a situation where the propagation pathconditions are degraded can communicate only in a single antennatransmission mode. Thus, it is necessary to cause terminal 200 toperform A-SRS transmission for single antenna transmission as soon aspossible. On the other hand, it is also possible to employ aconfiguration in which changing an SRS resource is not allowed upondetection of DCI that includes trigger information indicating an A-SRStransmission request and that is of a DCI format other than DCI format 0after detection of DCI that includes trigger information indicating anA-SRS transmission request and that is of DCI format 0. With thisconfiguration, the probability of terminal 200 performing erroneoustransmission after erroneously detecting DCI of a DCI format other thanDCI format 0 can be reduced.

Other Embodiments

(1) In the embodiments described above, the parameters defining an SRSresource include cyclic shift, comb, the number of RBs (or bandwidth),the RB position (or the frequency position where the SRS bandwidthstarts), frequency hopping pattern, the number of antennas, and/or thelike.

(2) In the embodiments described above, the parameters defining an SRSresource may include information about component carriers when carrieraggregation is applied to the communication system. Component carriersare also called cells. In addition, each terminal is configured with aset of component carriers (CCs) while each set of CCs includes onePrimary Cell (PCell) and one or more Secondary Cells (SCells). In thiscase, A-SRS transmission using PCell may be associated with DCI format 0while A-SRS trigger transmission using a SCell may be associated withDCI format 1A in the A-SRS transmission rule configuration information.

(3) In the embodiments described above, the starting position ofbandwidth, the bandwidth, Cyclic shift, and Comb number are used as thebasic configuration parameters for SRS resource configuration. However,the parameters are not limited to these parameters, and parameters otherthan the abovementioned ones may be included in the basic configurationparameters for SRS resource configuration.

(4) In the embodiments described above, when terminal 200 simultaneouslyreceives pieces of DCI each of which includes trigger informationindicating an A-SRS transmission request and each of which is of adifferent DCI format in the same subframe, terminal 200 may treat thesepieces of DCI as invalid. Accordingly, terminal 200 can be preventedfrom wrongly transmitting an A-SRS.

(5) In the embodiments described above, base station 100 may configurewhether or not to include SRS trigger information in DCI for eachterminal 200 and notify each terminal 200 of the result of configurationby RRC signaling. In this case, it is possible to reduce the number ofbits of DCI to be transmitted to terminal 200 not using an A-SRS (e.g.,terminal using only speech communication) or terminal 200 using anapplication not using an A-SRS can be reduced, thus making it possibleto reduce the overhead. In addition, base station 100 may configure thenumber of bits to represent SRS trigger information and notify terminal200 of the result of configuration via RRC signaling.

(6) In the embodiments described above, each terminal 200 is configuredto transmit an A-SRS in a common SRS subframe, but the present inventionis by no means limited to these embodiments, and each terminal 200 maybe configured to transmit an A-SRS in a specific SRS subframe. Stateddifferently, base station 100 may further configure terminal-specificSRS subframes from a group of common SRS subframes and allow terminals200 to perform A-SRS transmission in the terminal-specific SRSsubframes. In this case, the periodicity Np-ue of a terminal-specificSRS subframe is configured to be equal to or longer than the periodicityof the common SRS subframe, and the duration of the effective period totrigger an A-SRS in the same SRS subframe is Np_ue.

(7) The LTE system described above is often called 3GPP Release 8, andthe LTE-A system is often called Release 10. The LTE-A system hasbackward compatibility with the LTE system.

(8) When terminal 200 is configured with a plurality of componentcarriers (may be called cells), the technique described in theembodiments is applied only upon detection of A-SRS trigger informationon a first CC and then a different piece of DCI that includes A-SRStrigger information indicating an A-SRS transmission request on the samefirst CC within a single effective period, and upon detection of adifferent piece of DCI that includes A-SRS trigger informationindicating an A-SRS transmission request on a second CC different fromthe first CC, an A-SRS may be transmitted on the second CC according tothe trigger information included in the different piece of DCI.

(9) Component carriers are each defined by a physical cell ID and acarrier frequency, and often called a cell.

(10) It is also possible to configure the correspondence between DCIformats and pieces of information about SRS transmission power inaddition to the SRS resource parameters described above. For example, ina system configured to perform interference control in a coordinatedmanner between cells, an A-SRS is triggered using a DCI formatassociated with a low transmission power setting in a subframe whoseinterference with a different cell is preferably small, while an A-SRSis triggered using a DCI format associated with a high transmissionpower setting in a subframe whose interference with a different cell maybe large. Accordingly, A-SRS transmission power can be flexiblyconfigured without any increase in the control information.

(11) The SRS to be transmitted from each terminal 200 may be used forweight control (or precoding) of downlink antennas and/or the like inaddition to estimation of the propagation path conditions, uplink MCSconfiguration, frequency scheduling, and each antenna weight(directivity) control by base station 100. In this case, configuring adifferent DCI format with an SRS resource for uplink MCS configuration,frequency scheduling, and antenna weight control, and an SRS resourcefor downlink antenna weight control makes it possible to trigger anA-SRS in accordance with each application without any increase in thenumber of reporting bits.

(12) Each of the embodiments has been described with antennas, but thepresent invention can be applied to antenna ports in the same manner.

The term “antenna port” refers to a logical antenna including one ormore physical antennas. In other words, the term “antenna port” does notnecessarily refer to a single physical antenna, and may sometimes referto an antenna array formed of a plurality of antennas, and/or the like.

For example, 3GPP LTE does not specify the number of physical antennasforming an antenna port, but specifies an antenna port as a minimum unitallowing each base station to transmit a different reference signal.

In addition, an antenna port may be specified as a minimum unit formultiplication of precoding vector weighting.

(13) In the embodiments described above, uplink data is transmitted viaa Physical Uplink Shared Channel (PUSCH) while downlink data istransmitted via a Physical Downlink Shared Channel (PUSCH), but may betransmitted via another channel.

(14) The “SRS transmission execution rule” in each of the embodimentsdescribed above may be switched to another depending on the cellenvironment, the communication environment of the terminal, and/or thelike. In this case, base station 100 may broadcast informationindicating which one of the plurality of “SRS transmission executionrules” is to be used to all terminals 200 in the cell or may report theinformation to the terminals 200 individually (by RRC signaling).

(15) The above-noted embodiments have been described by examples ofhardware implementations, but the present invention can be alsoimplemented by software in conjunction with hardware.

In addition, the functional blocks used in the descriptions of theembodiments are typically implemented as LSI devices, which areintegrated circuits. The functional blocks may be formed as individualchips, or a part or all of the functional blocks may be integrated intoa single chip. The term “LSI” is used herein, but the terms “IC,”“system LSI,” “super LSI” or “ultra LSI” may be used as well dependingon the level of integration.

In addition, the circuit integration is not limited to LSI and may beachieved by dedicated circuitry or a general-purpose processor otherthan an LSI. After fabrication of LSI, a field programmable gate array(FPGA), which is programmable, or a reconfigurable processor, whichallows reconfiguration of connections and settings of circuit cells inLSI may be used.

Should a circuit integration technology replacing LSI appear as a resultof advancements in semiconductor technology or other technologiesderived from the technology, the functional blocks could be integratedusing such a technology. Another possibility is the application ofbiotechnology and/or the like.

The disclosure of the specification, the drawings, and the abstractincluded in Japanese Patent Application No. 2010-255843, filed on Nov.16, 2010, is incorporated herein by reference in its entirety.

INDUSTRIAL APPLICABILITY

The communication apparatus and SRS transmission method according to thepresent invention are useful in that they are capable of preventingdegradation in the system throughput by reducing the possibility ofoccurrence of a difference in understanding of the presence or absenceof SRS transmission or understanding of an SRS resource between the SRStransmission side and reception side.

REFERENCE SIGNS LIST

-   -   100 Base station    -   101 Configuration section    -   102, 103 Coding and modulation section    -   104 Transmission processing section    -   105, 208 RF transmitting section    -   106, 201 Antenna    -   107, 202 RF receiving section    -   108, 203 Reception processing section    -   109 Data receiving section    -   110 SRS receiving section    -   200 Terminal    -   204 Reference signal generating section    -   205 Data signal generating section    -   206 Transmission controlling section    -   207 Transmission signal forming section

The invention claimed is:
 1. A communication apparatus comprising: atransmitter, which, in operation, transmits, to a terminal, controlinformation indicating whether or not to request transmission of asounding reference signal (SRS) and indicating a transmission parameterof the SRS; and a receiver, which, in operation, receives the SRS thatis transmitted from the terminal based on the control information,wherein, when the transmitter transmits to the terminal first controlinformation that requests the transmission of the SRS and second controlinformation that requests the transmission of the SRS for the samesubframe and the same cell and that indicates a transmission parameterdifferent from the transmission parameter indicated by the first controlinformation, the receiver receives the SRS that is transmitted based onthe first control information.
 2. The communication apparatus accordingto claim 1, wherein the first control information and the second controlinformation are control information, which is detected within apredetermined period by the terminal.
 3. The communication apparatusaccording to claim 2, wherein the first control information is controlinformation, which is detected first within the predetermined period bythe terminal.
 4. The communication apparatus according to claim 2,wherein the first control information is control information, which isdetected last within the predetermined period by the terminal.
 5. Thecommunication apparatus according to claim 2, wherein the receiverreceives the SRS in first subframe n among subframes located at or aftera k-th subframe from a subframe in which the control informationrequesting the transmission of the SRS is detected by the terminal, thesubframes being configured based on periodicity Np and a subframeoffset, and the predetermined period is a period from subframen−(Np−k+1) until subframe n−k.
 6. The communication apparatus accordingto claim 1, wherein the transmitter transmits, to the terminal, only oneof the first and second control information within a predeterminedperiod.
 7. The communication apparatus according to claim 1, wherein thereceiver receives the SRS in a first subframe among subframes located ator after a k-th subframe from a subframe in which the controlinformation requesting the transmission of the SRS is detected by theterminal, the subframes being configured based on a periodicity and asubframe offset.
 8. The communication apparatus according to claim 1,wherein the transmission parameter includes one or more of a bandwidth,an RB position where the bandwidth starts, Cyclic Shift, transmissionComb, the number of antenna ports, and frequency hopping.
 9. Thecommunication apparatus according to claim 1, wherein the controlinformation is transmitted in one of a plurality of formats, and thetransmission parameter is identified based on the format.
 10. Thecommunication apparatus according to claim 1, wherein the SRS is anaperiodic SRS triggered by downlink control information.
 11. Acommunication apparatus comprising: a transmitter, which, in operation,transmits, to a terminal, control information indicating whether or notto request transmission of a sounding reference signal (SRS) andindicating a transmission parameter of the SRS; and a receiver, which,in operation, receives the SRS that is transmitted from the terminalbased on the control information, wherein, when the transmittertransmits to the terminal first control information that requests thetransmission of the SRS and second control information that requests thetransmission of the SRS for the same subframe and the same cell and thatindicates a transmission parameter different from the transmissionparameter indicated by the first control information, the receiverreceives no SRS.
 12. The communication apparatus according to claim 11,wherein the first control information and the second control informationare control information that is detected within a predetermined periodby the terminal.
 13. The communication apparatus according to claim 12,wherein the first control information is control information that isdetected first within the predetermined period by the terminal.
 14. Thecommunication apparatus according to claim 12, wherein the first controlinformation is control information that is detected last within thepredetermined period by the terminal.
 15. The communication apparatusaccording to claim 12, wherein the receiver receives the SRS in firstsubframe n among subframes located at or after a k-th subframe from asubframe in which the control information requesting the transmission ofthe SRS is detected by the terminal, the subframes being configuredbased on periodicity Np and a subframe offset, and the predeterminedperiod is a period from subframe n−(Np−k+1) until subframe n−k.
 16. Thecommunication apparatus according to claim 11, wherein the transmittertransmits, to the terminal, only one of the first and second controlinformation within a predetermined period.
 17. The communicationapparatus according to claim 11, wherein the receiver receives the SRSin a first subframe among subframes located at or after a k-th subframefrom a subframe in which the control information requesting thetransmission of the SRS is detected by the terminal, the subframes beingconfigured based on a periodicity and a subframe offset.
 18. Thecommunication apparatus according to claim 11, wherein the transmissionparameter includes one or more of a bandwidth, an RB position where thebandwidth starts, Cyclic Shift, transmission Comb, the number of antennaports, and frequency hopping.
 19. The communication apparatus accordingto claim 11, wherein the control information is transmitted in one of aplurality of formats, and the transmission parameter is identified basedon the format.
 20. The communication apparatus according to claim 11,wherein the SRS is an aperiodic SRS triggered by downlink controlinformation.
 21. A communication apparatus comprising: a transmitter,which, in operation, transmits, to a terminal, control informationindicating whether or not to request transmission of a soundingreference signal (SRS) and indicating a transmission parameter of theSRS; and a receiver, which, in operation, receives the SRS that istransmitted from the terminal based on the control information, wherein,when the transmitter transmits the control information twice or morewithin a predetermined period, the transmitter transmits the controlinformation, which indicates a same transmission parameter, within thepredetermined period.
 22. The communication apparatus according to claim21, wherein the receiver receives the SRS in first subframe n amongsubframes located at or after a k-th subframe from a subframe in whichthe control information requesting the transmission of the SRS isdetected by the terminal, the subframes being configured based onperiodicity Np and a subframe offset, and the predetermined period is aperiod from subframe n−(Np−k+1) until subframe n−k.
 23. Thecommunication apparatus according to claim 21, wherein the receiverreceives the SRS in a first subframe among subframes located at or aftera k-th subframe from a subframe in which the control informationrequesting the transmission of the SRS is detected by the terminal, thesubframes being configured based on a periodicity and a subframe offset.24. The communication apparatus according to claim 21, wherein thetransmission parameter includes one or more of a bandwidth, an RBposition where the bandwidth starts, Cyclic Shift, transmission Comb,the number of antenna ports, and frequency hopping.
 25. Thecommunication apparatus according to claim 21, wherein the controlinformation is transmitted in one of a plurality of formats, and thetransmission parameter is identified based on the format.
 26. Thecommunication apparatus according to claim 21, wherein the SRS is anaperiodic SRS triggered by downlink control information.
 27. Acommunication method comprising: transmitting, to a terminal, controlinformation indicating whether or not to request transmission of asounding reference signal (SRS) and indicating a transmission parameterof the SRS; and receiving the SRS that is transmitted from the terminalbased on the control information, wherein, when the transmittingincludes transmitting, to the terminal, first control information thatrequests the transmission of the SRS and second control information thatrequests the transmission of the SRS for the same subframe and the samecell and that indicates a transmission parameter different from thetransmission parameter indicated by the first control information, thereceiving includes receiving the SRS that is transmitted based on thefirst control information.
 28. A communication method comprising:transmitting, to a terminal, control information indicating whether ornot to request transmission of a sounding reference signal (SRS) andindicating a transmission parameter of the SRS; and receiving the SRSthat is transmitted from the terminal based on the control information,wherein, when the transmitting includes transmitting, to the terminal,first control information that requests the transmission of the SRS andsecond control information that requests the transmission of the SRS forthe same subframe and the same cell and that indicates a transmissionparameter different from the transmission parameter indicated by thefirst control information, the receiving includes receiving no SRS. 29.A communication method comprising: transmitting, to a terminal, controlinformation indicating whether or not to request transmission of asounding reference signal (SRS) and indicating a transmission parameterof the SRS; and receiving the SRS that is transmitted from the terminalbased on the control information, wherein, when the control informationis transmitted twice or more within a predetermined period, thetransmitting includes transmitting the control information, whichindicates a same transmission parameter, within the predeterminedperiod.